Semiconductor laser diodes, for example, have been used as multiplexed sources. In these the wavelength of operation depends upon the composition of the active layer material. Diodes thus with different active layer conpositions have been used to generate the different wavelengths. The finite spectral width typical of such diodes, and the manufacturing tolerance on active layer composition, however, usually would limit the attainable minimum channel spacing to circa 30 nm. Closer channel spacing can be attained by using single frequency type lasers--for example distributed feedback (DFB) lasers or cleave-coupled-cavity (C.sup.3) lasers. Here, however, elaborate control is required to maintain the emission wavelength of each laser correct and in tune with the multiplexer used to combine all of the outputs from the different lasers.
By way of illustration, a typical apparatus configuration is presented in FIG. 1. This has the form of a ten-channel multiplexing device 1. In this arrangement, ten fixed-wavelength lasers L1 to L10, are combined in an input linear array 3 by means of coupling fibres 5. This array 3, which also includes a common output fibre 7 is located in the focal plane of Littrow mount optical assembly 9 comprised of a collimating and focussing lens 11 and a dispersive member, a reflective grating 13. Light emissions from the individual lasers L1 to L10 are collimated, diffracted and refocussed onto the common output fibre 7. It will be appreciated that in this arrangement, the emission wavelengths and array geometry must be carefully matched to ensure that all emissions are diffracted to a common focus.
Such an arrangement, as described above, is detailed in the following article: "68.3 km transmission with 1.37 Tbit km/s capacity using wavelength division multiplexing of ten single frequency lasers at 1.5 .mu.m", by Olsson, N. A. et al., published in Electronics Letters Volume 2, Number 3 pages 105-106 (1985). There, the authors describe an arrangement in which the selected ten lasers were of the hetero-epitaxially ridge overgrown (HRO) distributed feedback (DFB) laser type. These were made with a second-order diffraction grating and with both facets cleaved. The wavelength distribution of the lasers, which were selected from wafers of different design wavelengths, was between 1.529 and 1.561 .mu.m.
Wafer selection, as above, is an expensive and time-consuming procedure and clearly is not commercially practical for mass production. Long term stability also is likely to be a severe problem in this type of arrangement.